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Transcript of “HEAT PIPE”
SEMINAR REPORTON
“HEAT PIPEHEAT PIPE”
SUBMITED BY
Mr. Balaji M.Chavan
Under the guidance of
Prof. U. A. Shinde. Prof. R.G.Biradar.
Co-Guide Guide
Department of Mechanical Engineering
S. V. E. R.I.’s
COLLEGE OF ENGINEERING, PANDHARPUR
2003-2004
1
HEAT PIPE
S. V. E. R.I.’s
COLLEGE OF ENGINEERING, PANDHARPUR
CERTIFICATECERTIFICATE
This is to certify that the seminar report entitled
“ HEAT PIPE ”“ HEAT PIPE ”
has been carried out
By
Mr. BALAJI M.CHAVAN
of B.E. ( MECHANICAL ) class in partial fulfillment
for award of Bachelor’s Degree in Mechanical
Engineering as per curriculum laid down by the
SHIVAJI UNIVERSITY, KOLHAPUR during the
academic year 2003-2004.
(Prof. U. A. SHINDE) (Prof. R. G.BIRADAR) Co-Guide Guide
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HEAT PIPE
(Prof. S. A.PATIL) (Prof. B. P. RONGE) H.O.D. Principal
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HEAT PIPE
ABSTRACT
The transfer of the heat energy by conduction using solid
material is essentially limited by thermal conductivity of that material. As
the thermal conductivity increases cost of the material also increases
hence it become costly. Because of the thermal energy is being
transported by evaporation–condensation process rather than conduction.
The heat pipe can transfer the heat much more effectively than the solid
conductor of the same cross-section in practice conduction of heat by heat
pipe may be several hundred (500) times that the best available metal
conductor such as copper.
Heat pipe system provides the maximum effective heat sink
surface area with the minimum volume demand. A heat pipe heat sink is a
passive cooling device that requires no moving parts, and operates silently
and reliably. Additionally, heat pipe technology is emerging as a cost-
effective thermal design solution. This paper explains the operation of heat
pipes and its various applications.
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HEAT PIPE
INDEXINDEX
SR.NO. CONTENTS PAGE NO.
1 HISTORY 1
2 INTRODUCTION 2
3 WORKING 3
4 DESIGN CONSIDERATIONS 4
5 OPERATING LIMITATIONS 10
6 APPLICATIONS 13
7 ADVANTAGES 19
8 DISADVANTAGES 20
9 CONCLUSION 21
10 REFERENCES 22
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HEAT PIPE
HISTORY
The development of the heat pipe originally started with Angier
March Perkins who worked initially with the concept of the working fluid
only in one phase (he took out a patent in 1839 on the hermetic tube
boiler which works on this principle). Jacob Perkins (descendant of
Angier March) patented the Perkins Tube in 1936 and they became
widespread for use in locomotive boilers and baking ovens. The Perkins
Tube was a system in which a long and twisted tube passed over an
evaporator and a condenser, which caused the water within the tube to
operate in two phases. Although these early designs for heat transfer
systems relied on gravity to return the liquid to the evaporator (later
called a thermosyphon), the Perkins Tube was the jumping off point for
the development of the modern heat pipe. The concept of the modern
heat pipe, which relied on a wicking system to transport the liquid
against gravity and up to the condenser, was put forward by R.S.
Gaugler of the General Motors Corporation. According to his patent in
1944, Gaugler described how his heat pipe would be applied to
refrigeration systems. Heat pipe research became popular after that and
many industries and labs including Los Alamos, RCA, the Joint Nuclear
Research Centre in Italy, began to apply heat pipe technology their
fields. By 1969, there was a vast amount of interest on the part of NASA,
Hughes, the European Space Agency, and other aircraft companies in
regulating the temperature of a spacecraft and how that could be done
with the help of heat pipes. There has been extensive research done to
date regarding specific heat transfer characteristics, in on to the analysis
faddist various material properties and geometries
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HEAT PIPE
Introduction
What is a Heat Pipe?
A heat pipe is a simple device that can quickly transfer heat from
one Point to another. By means of evaporation & condensation of fluid in
a sealed system they are often referred to as the "superconductors" of
heat as they possess an extra ordinary heat transfer capacity & rate with
almost no heat loss.
It consists of a sealed aluminum or copper container whose inner
surface have a capillary wicking material. The working fluid is placed
inside it &it is highly evacuated. Because of that the working fluid is
virtually in a state of liquid-vapour equilibrium. consequently, a slight
increase in temperature will cause it to boil &evaporate The central
portion of it is heavily insulated on the outside. One end of pipe is known
as heating end (evaporator) where heat is absorbed & the other end is
known as cooling end (condenser) where heat is given out.
A heat pipe is similar to a thermosyphon. It differs from a
thermosyphon by Virtue obits ability to transport heat against gravity by
an evaporation –condensation cycle with the help of porous capillaries
that form the wick. The wick provides the capillary driving force to return
the condensate to the evaporator. The quality and type of wick usually
determines the performance of the heat pipe, for this is the heart of the
product. different types of wicks are used depending on the application
for which the heat pipes being used
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HEAT PIPE
Working
Inside the container is a liquid under its own pressure, that enters the
pores of the capillary material, wetting all internal surfaces. Applying
heat at any point along the surface of the heat pipe causes the liquid at
that point to boil and enter a vapor state. When that happens, the liquid
picks up the latent heat of vaporization. The gas, which then has a
higher pressure, moves inside the sealed container to a colder location
where it condenses. Thus, the gas gives up the latent heat of
vaporization and moves heat from the input to
Fig. Working of the heat Pipe
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HEAT PIPE
Design Considerations
The three basic components of a heat pipe are:
1. the container
2. the working fluid
3. the wick or capillary structure
The choice of each component has marked effect on the working
Performance of heat pipe and therefore proper selection of each
Component is very important in design of heat pipe. Following explanation
is given below
1. Container
The function of the container is to isolate the working fluid from the
outside environment. It has to therefore be leak-proof, maintain the
pressure differential across its walls, and enable transfer of heat to take
place from and into the working fluid.
Selection of the container material depends on many factors. These
are as follows:
Compatibility (both with working fluid and external environment)
Strength to weight ratio
Thermal conductivity
Ease of fabrication, including welding, machine ability and ductility
Porosity
Wettability
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Most of the above are self-explanatory. A high strength to weight ratio
is more important in spacecraft applications. The material should be non-
porous to prevent the diffusion of vapor. A high thermal conductivity
ensures minimum temperature drop between the heat source and the
wick.
Material used for heat pipe is stainless steel; copper; aluminum,
ceramic material, glass etc depending on temperature range Usually they
are in tubular form but it can be constructed in any shape such as Y, T, U,
etc. depending upon requirement Effect of length & diameter on The heat
transfer capacity of heat pipe is specified by the “axial power
rating”(APR)Which is energy moving axially along the pipe larger the
diameter; greater will be the APR for the given length ;a 5 mm dia.&15cm
long pipe has an APR of 75 watts which increases to 500 watts if dia. Is
increased to20mm. The physical size of heat pipe that have been
operated successfully range from 6 mm to 150mm in dia. & up to 5 miter
long in length
2. Working fluid
A first consideration in the identification of a suitable working fluid
is the Operating vapour temperature range. Within the approximate
temperature band, several possible working fluids may exist, and a variety
of characteristics must be examined in order to determine the most
acceptable of these fluids for the application considered. The prime
requirements are:
compatibility with wick and wall materials
good thermal stability
wettability of wick and wall materials
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HEAT PIPE
vapour pressure not too high or low over the operating temperature
range
High latent heat
High thermal conductivity
Low liquid and vapor viscosities
High surface tension
Acceptable freezing or pour point
The selection of the working fluid must also be based on
thermodynamic considerations which are concerned with the various
limitations to heat flow occurring within the heat pipe like, viscous, sonic,
capillary, Entrainment and nucleate boiling levels. In heat pipe design, a
high value of surface tension is desirable in order to enable the heat pipe
to operate against gravity and to generate a high capillary driving force. In
addition to high surface tension, it is necessary for the working fluid to wet
the wick and the container material i.e. contact angle should be zero or
very small. The vapor pressure over the operating temperature range
must be sufficiently great to avoid high vapor velocities, which tend to
setup large temperature gradient and cause flow instabilities.
A high latent heat of vaporization is desirable in order to transfer
large amounts of heat with minimum fluid flow, and hence to maintain low
pressure drops within the heat pipe. The thermal conductivity of the
working fluid should preferably be high in order to minimize the radial
temperature gradient and to reduce the possibility of nucleate boiling at
the wick or wall surface..
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HEAT PIPE
Tabulated below are a few mediums with their useful
ranges of temperature.
MEDIUM MELTING
PT. ( C)
BOILING
PT. AT
ATM.
PRESSURE
( C)
USEFUL
RANGE
( C)
Helium
Nitrogen
Ammonia
Acetone
Methanol
FlutecPP2
Ethanol
Water
Toluene
Mercury
Sodium
Lithium
Silver
-271
-210
-78
-95
-98
-50
-112
0
-95
-39
98
179
960
-261
-196
-33
57
64
76
78
100
110
361
892
1340
2000
-271 to -269
-203 to -160
-60 to 100
0 to 120
10 to 130
10 to 160
0 to 130
30 to 200
50 to 200
250 to 650
600 to 1200
600 to 1200
1000 to 1800
1000 to 1800
1800 to 2300
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3. Wicks or Capillary Structure
It is a porous structure made of materials like steel, aluminum,
nickel or copper in various ranges of pore sizes. They are fabricated using
metal foams, and more particularly felts, the latter being more frequently
used. By varying the pressure on the felt during assembly, various pore
sizes can be produced. By incorporating removable metal mandrels, an
arterial structure can also be molded in the felt.
Fibrous materials, like ceramics, have also been used widely. They
generally have smaller pores. The main disadvantage of ceramic fibers is
that, they have little stiffness and usually require a continues support by a
metal mesh. Thus while the fiber itself may be chemically compatible with
the working fluids, the supporting materials may cause problems. More
recently, interest has turned to carbon fibers as a wick material. Carbon
fiber filaments have many fine longitudinal grooves on their surface, have
high capillary pressures and are chemically stable. A number of heat pipes
that have been successfully constructed using carbon fibre wicks seem to
show a greater heat transport capability.
The prime purpose of the wick is to generate capillary pressure to
transport the working fluid from the condenser to the evaporator. It must
also be able to distribute the liquid around the evaporator section to any
area where heat is likely to be received by the heat pipe. Often these two
functions require wicks of different forms. The selection of the wick for a
heat pipe depends on many factors, several of which are closely linked to
the properties of the working fluid.
The maximum capillary head generated by a wick increases with
decrease in pore size. The wick permeability increases with increasing
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HEAT PIPE
pore size. Another feature of the wick, which must be optimized, is its
thickness. The heat transport capability of the heat pipe is raised by
increasing the wick thickness. The overall thermal resistance at the
evaporator also depends on the conductivity of the working fluid in the
wick. Other necessary properties of the wick are compatibility with the
working fluid and wettability. The most common types of wicks that are
used are as follows:
Sintered Powder
This process will provide high power handling, low temperature
gradients and high capillary forces for anti-gravity applications. The
photograph shows a complex sintered wick with several vapor channels
and small arteries to increase the liquid flow rate. Very tight bends in the
heat pipe can be achieved with this type of structure.
Grooved Tube
The small capillary driving force generated by the axial grooves is
adequate for low power heat pipes when operated horizontally, or with
gravity assistance. The tube can be readily bent. When used in
conjunction with screen mesh the performance can be considerably
enhanced.
Screen Mesh
This type of wick is used in the majority of the products and
provides readily variable characteristics in terms of power transport and
orientation sensitivity, according to the number of layers and mesh counts
used.
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HEAT PIPE
Operating Limitations
Since the heat pipe benefits from the phase change of the working
fluid, the thermodynamics of the process are critical. The operation of the
heat pipe is limited by several operating phenomena. Each of these
limitations is dependant on the wick structure, working fluid, temperature,
orientation, and size of the heat pipe. Below is a brief description of each
of the limitations:
Capillary Limit
The wick structure of the heat pipe generates a capillary pressure,
which is dependent on the pore radius of the wick and the surface tension
of the working fluid. The capillary pressure generated by the wick must be
greater than the sum of the gravitational losses, liquid flow losses through
the wick, and vapor flow losses. The liquid and vapor pressure drops are a
function of the heat pipe and wick structure geometry (wick thickness,
effective length, vapor space diameter, etc) and the fluid properties (latent
heat, density, viscosity, etc). A critical heat flux exists that balances the
capillary pressure with the pressure drop associated with the fluid and
vapor circulation. For horizontal or against gravity (evaporator at a higher
elevation than the condenser), the capillary limit is the heat pipe limit. For
gravity-aided orientations, the capillary limitation may be neglected, and
the flooding limit may be used if the heat pipe can have an excess fluid
charge.
Boiling Limit
As more heat is applied to the heat pipe at the evaporator, bubbles
may be formed in the evaporator wick. The formation of vapor bubbles in
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HEAT PIPE
the wick is undesirable because they can cause hot spots and obstruct the
circulation of the liquid.
As the heat flux is increased, more bubbles are formed. At a certain
heat flux limit, the bubble formation completely blocks the liquid flow. This
limitation is associated to a radial heat flux (heat is applied to the
perimeter of the heat pipe). The boiling limitation is typically a high
temperature phenomenon. Heat flux limitations for various wick structures
should be used for design criteria. Sintered powder metal wick structures
have significantly more surface area, and can therefore handle higher heat
fluxes. Conservative values are 50 W/cm2 for powder metal wicks, 10
W/cm2 for screen wicks, 5 W/cm2 for bare wall thermosyphons.
Sonic Limit
In a heat pipe of constant vapor space diameter, the vapor flow
accelerates and decelerates because of the vapor addition in the
evaporator and the vapor removal in the condenser. The changes in vapor
flow also change the pressures along the heat pipe. As more heat is
applied to the heat pipe, the vapor velocities generally increase. A choked
flow condition will eventually arise, where the flow becomes sonic. At this
point, the vapor velocities can not increase and a maximum heat transport
limitation is achieved. The heat flux that results in choked flow is
considered the sonic limit. The addition of more heat will result in an un
proportional increase in the heat pipe temperature delta by an increase in
the evaporation temperature. This phenomenon is self-correcting as the
heat pipe warms up. An additional benefit of the high vapor velocities is
the very quick response to heat input.
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HEAT PIPE
Entrainment Limit
Since the vapor and the liquid move in opposite directions in a heat
pipe, a shear force exists at the liquid-vapor interface. If the vapor velocity
is sufficiently high, a limit can be reached at which the liquid will be torn
from the pores of the wick and entrained in the vapor. When enough fluid
is entrained in the vapor that the condensate flow is stopped, abrupt dry-
out of the wick at the evaporator results. The corresponding heat flux that
results in this phenomenon is called the Entrainment Limit. The
Entrainment Limit is typically not the bounding value.
Flooding Limit
The flooding limit is only applicable to gravity aided orientations
with excess fluid. The wick structure is saturated and the excess fluid
results in a “puddle” flow on the surface of the wick structure. The flooding
limit, similar to the entrainment, occurs when high vapor velocities
preclude the fluid that is flowing on the surface of the wick to return to the
evaporator. The vapor shear hold up prevents the condensate from
returning to the evaporator and leads to a flooding condition in the
condenser section. This causes a partial dry-out of the evaporator, which
results in wall temperature excursions or in limiting the operation of the
system. By increasing the heat flux above the flooding limit, it is possible
to achieve liquid flow reversal leading to:
1) The accumulation of liquid in the condenser.
2) The accumulated liquid falling to the evaporator due to gravity.
3) The reestablishment of a flow reversal situation.
4) The repeat cycle of flooding and normal flow.
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Fig. Cooking pin
HEAT PIPE
Applications
Heat pipe has been, and is currently being, studied for a variety of
applications, covering almost the entire spectrum of temperatures
encountered in heat transfer processes. Heat pipes are used in a wide
range of products like air-conditioners, refrigerators, heat exchangers,
transistors, capacitors, etc. Heat pipes are also used in laptops to reduce
the working temperature for better efficiency. Their application in the field
of cryogenics is very significant, especially in the development of space
technology. We shall now discuss a brief account of the various
applications of heat pipe technology.
1.Heat pipe used for extracting solar energy
Solar radiations are focused on the heat pipe at the evaporator
side by a parabolic reflector. Heat pipe leading from the reflector could be
coupled to steam raising boilers or end of heat pipe directly used as
cooking plate.
A recent use of heat pipe in kitchen is a cooking pin. This is a
simply a heat pipe which when inserted into joint of meat cooking in an
oven, speedup the rate of heat flow. Thus saving the time &fuel &yielding
a more uniform roast. A particular type is shown below.
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HEAT PIPE
2. Heat dissipation device
In the electronic devices, various components (such as I.C.s,
capacitors etc.) generates the heat .The performance of these
components decreases with increase in temperature. Hence heat
dissipation is necessary. For this fan is directly placed over the device &
hence occupy most valuable real estate.
Here heat pipe play very imp. role in theses cases heat pipe is
employed to transfer the heat from small area available on the component
to a larger area where the heat is released to atmosphere. This is
achieved by keeping the evaporator of the heat pipe .In contact with the
electronic device &the condenser gives the heat to the large surface area.
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Chip which to be cooled
Heat Pipe
Fig .Heat Deceptions Device
HEAT PIPE
3.Temperature Control device
In some applications, it is required to maintain the temperature of
device to specific value, heat pipe can be used for the same application
with small modifications A reservoir containing a non-condensable gas is
connected to the heat removal end (the condenser) of the heat pipe. This
gas forms an interface with the vapor & “chokes off” part of condenser
area of heat pipe. As the temperature of the device increases the vapor
pressure inside the pipe & the non-condensable gas is forced back into
reservoir, thereby opening up additional condenser area to give more heat
flow. Thus finally reduces the temperature of the device& vice-versa
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Condenser Length
Fig . Temperature Control Device.
Non Condensable Gas
HEAT PIPE
4. Dry drilling
For the proper drilling the temperature of the drill must be low; for
this we are using coolant which Is very costly & goes waste after use .By
using heat pipe we can solve this problem .As here liquid is not used for
cooling, it is called dry drilling. Fig shows distribution of heat in drill & use
of heat pipe
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Distribution of heat in drill
Heat pipe used in drill shank
HEAT PIPE
5. Injection moulding and die casting
Fig. Heat pipe used for Injection Moulding &Die Casting
Heat Pipes are widely used for improving cooling efficiency of
injection moulds and Die casting dies all over the world. This method of
cooling has helped reduce cycle time, reduce rejection and improves
quality of product. Sketches given inside the folder describe various
applications where one can confidently use Heat Pipes. These are taken
from actual examples of moulds, which are in production all over India. In
conventional water-cooling, effectiveness of water cooling goes down due
to rusting, blocking of cooling channels. In case of Heat Pipes since water
is not circulated directly in the core, cooling efficiency remains the same
throughout the life of the mould.
Good Reasons To Use Heat Pipes:
1. Reduce cycle time 2. Eliminate hot spots
3. Reduce wastage 4. Improve product quality
5. Increase mould life 6. Eliminate core clogging
7. Cut mould and Moulding costs 8. Upgrade old moulds
9. Use damaged moulds 10. Eliminate hot spots
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HEAT PIPE
6. Heat Pipe Heat Exchanger
Typical finned air-to-air Heat Pipe heat exchangers comprise of
number of tubular gravity assist. Air to air Heat Pipe teed finned Heat
Pipes arranged in staggered pitch, depending upon the application. One of
the advantages of the Heat Pipe Heat Exchanger is its ability to operate
without cross contamination between the two gas streams. Use of Gravity
Assisted Heat Pipe complies orientation evaporator above condenser.
7. Application for I.C.engine
Heat pipe uses energy of exhaust gas for homogeneous vaporizing
the fuel, which is coming after carburettor. Therefore fuel consumption is
decreased.
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HEAT PIPE
Advantages of the heat pipes
Rate of heat transfer is very high than the solid material.
It has no moving parts hence maintains is not required
It can transmit heat over the appreciable distance without loss of
the heat (i. e isothermal). And thus permitting separation of the
heat source and sink
It require no power source to accomplish this function
It can transfer the heat where a very low temperature difference is
available in between source and sink.
It is ideal device for removing the heat from a concentrated heat
source such as thermocore.
It is rugged like any piece of pipe or tube and has no any wearing
part hence it has long life.
The absence of the gravity does not affect the operation of the heat
pipe determinately liquid flow does not depend upon gravity
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HEAT PIPE
Disadvantages
Like any other practical devices, heat pipe has also
disadvantages as listed below:
Undesired increase in point-to-point temperature differential along
the heat pipe can lead to damage to evaporator section
Length of heat pipe is limited
Design is complicated
The cost of a given heat pipe will tend to reach a minimum in the
temperature range of 70 to 120 degree C. But above &below this
range, total cost of heat pipe will be more
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HEAT PIPE
Conclusion
Now days we requires the transfer of heat from one place
(source) to other place (sink) very fatly, without loss of energy &
economically. These requirements are fulfilled by heat pipe. Presently it
plays very important role in thermal science .it is widely used all over the
world for improving efficiency & rate of heat transfer. It is presently used in
space technology, thermal power stations, home applications etc has. It
has very bright future.
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HEAT PIPE
References BOOKS
Heat & Mass Transfer………….S.C.Arora & S. Domkundwar
Heat Transfer…………………...Pavaskar.
WEBSITES
www.google.com
www.heat pipe .com
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